1. Field of the Invention
This invention relates to a flat antenna, and more particularly to an improved feeding method suitable for a coaxial cable connected to the feeder point of a flat antenna.
2. Description of the Related Art
Recently, simple flat antennas which can be manufactured at low costs have been developed as widespread antennas for the mobile communication system.
The flat antenna or thin antenna is configured, for example, by disposing a patch conductor cut to a predetermined size over a grounded conductive plate through a dielectric material. This structure allows an antenna with high sensitivity over several GHz rf waves to be fabricated in a relatively simple structure. Such an antenna can be easily mounted to appliances.
However, a problem arises in using the flat antenna resonating at a receive frequency and in designing a radiation resistance, or the impedance at the feeder point having a real-part component. That is, when received rf waves are taken out of the antenna or the coaxial cable for supplying transmission power to the antenna is connected to the patch antenna, various kinds of machining are required to match the impedance of the feeder point.
The above-mentioned problem will be described below with reference to FIG. 5. FIG. 5(a) is a cross sectional view partially illustrating a flat antenna. Referring to FIG. 5(a), numeral 10 represents a patch antenna section made of a conductive plate sized so as to resonate to a received frequency, 11 represents a dielectric material, and 12 represents a grounded conductive plate.
Numeral 13 represents a center conductor of a coaxial cable disposed to feed power to the patch antenna section 10. The outer conductor of the coaxial cable is grounded within the opening 12A of the grounded conductive plate 12.
The dielectric material 11 with a high dielectric constant is used to miniaturize the antenna. A thick dielectric material 11 of a large thickness generally provides a higher receive sensitivity and a wider receive band.
However, the center conductor 13 inserted into the dielectric material 11 induces an inductive impedance component L at the opening. In designing, the impedance at the feeder point of the patch antenna resonating at a specific receive frequency is usually set to have only a radiation resistance component. Hence, in order to cancel the inductive impedance L added to the terminal impedance of the coaxial cable, the center conductor 13 of the coaxial cable is disposed to pass though the feeder point of the patch antenna, as shown in FIG. 5, and the tip thereof is connected to a chip conductor 15. The coaxial cable is matched with the patch antenna by means of the capacitive impedance C formed between the chip conductor 15 and the patch antenna section 10.
FIG. 5(b) shows a circular patch antenna in which like elements are represented with like numerals as shown in FIG. 5(a). In the case of the conventional structure shown in FIG. 5(b), in order to cancel the inductive impedance L added by the center conductor 13 of the coaxial cable penetrating the dielectric material, an island conductor 10B insulated from the patch antenna is disposed at the feeder point of the patch antenna 10A. The patch antenna 10A is matched with the coaxial cable by means of the capacitance C defined by the gap t between the island conductor 10B and the patch antenna 10A.
Referring to FIG. 5(c), an insulating material 15 is disposed between the patch antenna portion 10 and the dielectric material 11. The center conductor 13 of the coaxial cable is connected to the chip conductor 16 disposed underneath the insulating layer 15. Thus, the matching configuration which cancels the inductive impedance L is provided by adding the capacitive impedance C between the chip conductor 16 and the patch antenna section 10.
As described above, in order to feed power with the coaxial cable, the conventional flat antenna is electrically matched to cancel the inductive impedance L of the center conductor penetrating the dielectric material 12. Hence, the problem is that the patch antenna section must be machined to some degree so that the structure of the flat antenna is complicated.